Skin is exposed to damage resulting from various sources, including both environmental factors and biochemical processes. Oxidative processes damage proteins, lipids, and other cellular components necessary to maintain the health and appearance of skin, resulting in skin changes, such as skin aging (e.g., age spots), hyperpigmentation, UV damage, lines, wrinkles, uneven skin texture (e.g., cellulitis), etc. Oxidative damage to the skin and its more detailed causes are listed in Miyachi, Y: “Skin diseases associated with oxidative injury,” Fuchs J, Packer L (eds.), OXIDATIVE STRESS IN DERMATOLOGY, Marcel Dekker, New York, pp. 323-331 (1993).
The damaging effects of the UV part of solar radiation on the skin are generally known. While rays having a wavelength which is less than 290 nm (the UVC range), are absorbed by the ozone layer in the earth's atmosphere, rays in the range between 290 nm and 320 nm (the UVB range), cause an erythema, simple sunburn or even more or less severe burns. The narrower range around 308 nm is given as a maximum for erythema activity of sunlight. For protection against UVB radiation, numerous compounds are known, in which they are derivatives of 3-benzylidene camphor, 4-aminobenzoic acid, cinnamic acid, salicylic acid, benzophenone and also 2-phenylbenzimidazole. Also, for the range between about 320 nm and about 400 nm (the UVA range) it is important to have filter substances available, since its rats may cause reactions in light-sensitive skin. It has been proven that UVA radiation leads to damage of the elastic and collagenic fibres of the connective tissue, which allows the skin to age prematurely, and that it is to be regarded as a cause of numerous phototoxic and photoallergic reactions. The damaging influence of UVB radiation may be amplified by UVA radiation. It has also been proven that consumption of lipophilic antioxidants, for example, alpha-tocopherol, is triggered in the skin by UVA and UVB radiation (Thiele et al., J. Invest. Dermatol. 100, p. 756 ff. (1998)).
Further, UV radiation is ionizing radiation. Hence, there is the risk that ionic species are produced on UV exposure, which then in turn are able to intervene oxidatively in the biochemical processes.
For protection against the rays of the UVA range, certain derivatives of dibenzoylmethane have therefore been used, the photostability of which (Int. J. Cosm. Science 10, 53 (1988)) is not provided to an adequate extent. UV radiation, however, may also lead to photochemical reactions, wherein then the photochemical reaction products intervene in the skin mechanism.
Predominantly such photochemical reaction products are free radical compounds, for example hydroxyl radicals. Also, undefined free radical photoproducts, which are produced in the skin itself, may trigger uncontrolled side reactions due to their high reactivity. Singlet oxygen, a non-free radical excited state of the oxygen molecule, however, may occur in UV irradiation, short-lived epoxides and many others. Singlet oxygen, for example, is characterized with respect to the normally existing triplet oxygen (free radical base state) by increased reactivity. Nevertheless, excited, reactive (free radical) triplet states of the oxygen molecule also exist. Furthermore, there is the occurrence of lipid peroxidation products, such as hydroperoxides and aldehydes, wherein first in turn free radical chain reactions may be triggered and to which overall cytotoxic properties have to be ascribed (Michiels and Ramacle, Toxicology, 66, 225 ff. (1990)). Lipid peroxidation is an oxidative process that degrades lipids, wherein free radicals steal electrons from the lipids in cell membranes, causing oxidative stress and cell damage.
Light-sensitive skin includes the disorder photodermatoses (photosensitive eruptions). Further designations for the polymorphic light-dermatosis are PLD, PLE, Mallorca Acne and a plurality of further designations, as are given in the literature (e.g., A. Voelckel et al., Zentralblatt Hautund Geschlechtskrankheiten (1989), 156, p. 2).
Erythematous skin symptoms also occur as concomitant symptoms in certain skin diseases or skin irregularities. For example, the typical rash in the clinical picture of acne is regularly reddened to a greater or lesser extent.
In order to prevent these reactions, additional antioxidants and/or free radical absorbers/scavengers may be incorporated in cosmetic or dermatological formulations. Antioxidants are substances that scavenge free radicals and prevent oxidation processes or prevent the auto-oxidation of fats containing unsaturated compounds. Antioxidants used in the field of cosmetics and pharmacy are, for example, alpha-tocopherol, in particular in the form of alpha-tocopheryul acetate, sesamol, colic acid derivatives, butylhydroxy anisole, butylhydroxy toluene, and idebenone. Antioxidants are mainly used as protective substances against the decay of the compositions containing them. However, it is known that undesirable oxidation processes may also occur in the human and animal skin. Such processes play a considerable part in skin aging. Thus, antioxidants and/or free radical absorbers may additionally be incorporated into cosmetic formulations to treat or prevent damage caused by oxidative and degenerative biochemical processes. It has been proposed to use vitamin E (U.S. Pat. Nos. 4,144,325 and 4,248,861), a substance having known anti-oxidative action in sunscreen formulations, but even here the action achieved remains far below that hoped for. Tocopherol (a vitamin E antioxidant), for example, degrades to form pro-oxidative products.
Idebenone (6-(10-hydroxydecyl)-2,3-dimethoxy-5-methyl-1,4-benzoquinone) has been used previously to treat skin changes. For example, U.S. Pat. No. 6,756,045, describes the use of idebenone as a topical composition for treating skin changes.